The present invention relates generally to a scanning optical apparatus for use in image processing apparatuses such as laser printer, facsimile, or digital copying apparatus. More particularly, the invention relates to a scanning optical apparatus using a liquid crystal lens as a vari-focal lens for imaging a light beam such as a laser beam collimated by the collimating optical system.
There is known a scanning optical apparatus in which a light beam collimated by a collimating optical system is imaged so as to form a linear image which is then deflected by a rotating polygonal mirror having a plurality of light reflection surfaces and imaged on a target surface through a second imaging optical system.
Generally, the scanning optical apparatus utilizing such a polygonal mirror has a problem in that a scanning position on the target surface tends to fluctuate in the sub-scanning direction due to deviations in parallelism of the polygonal mirror surfaces in relation to the rotational axis of the polygonal mirror. In order to correct the deviations, the first imaging optical system is constructed to form a linear image in the vicinity of the mirror surfaces, and the second imaging optical system is constructed to maintain a conjugate relationship in view of geometrical optics between each of the reflection surfaces and the target surface in a sub-scanning direction perpendicular to a main scanning direction.
Accordingly, the second imaging optical system tends to have a property as an anamorphic optical system in that the power of component of a light beam in the sub scanning direction is greater than that of a component thereof in the main scanning direction, and thus a great curvature of field tends to occur in the sub-scanning direction.
The curvature of field in the sub-scanning direction causes the size of a beam spot on the target surface in the sub-scanning direction to be varied with the scanning position thereof. The fluctuation of such a beam spot size causes the high density scanning to be disturbed.
In order to correct deviations in parallelism of the polygonal mirror surfaces in relation to the rotational axis of the polygonal mirror, many attempts to increase the accuracy of the second imaging optical system have been made. However, such attempts have been accompanied with significant difficulties in manufacturing of the optical system.
A scanning optical apparatus for solving the above-mentioned problem has been proposed and disclosed in Japanese Patent Application No. 63-212057, filed by the same applicant, in which a vari-focal liquid crystal lens is used as a first imaging optical system. In this apparatus, focal length of the vari-focal liquid crystal lens is adapted to be varied with the timing of scanning movement of the polygonal mirror surfaces so as to correct the curvature of field in the sub-scanning direction.
In the scanning optical apparatus using the above-mentioned vari-focal liquid crystal lens, an image corresponding to the linear image formed by the first imaging optical system is formed on the target surface due to a geometrically conjugate relationship between each of the mirror surfaces and the target surface in a sub-scanning direction. When the focal length of the first imaging optical system is varied, the linear image formed by the first imaging optical system is shifted along the optical axis thereof. Accordingly, the imaging position of the second imaging optical system in the sub-scanning direction is shifted with changes in the position of the linear image formed by the first imaging optical system in proportion to the longitudinal magnification of the second imaging optical system. This phenomenon is utilized to correct the curvature of field in the sub-scanning direction.
By the way, liquid crystal lens generally have a disadvantage, as described in "VARI-FOCAL LENS WITH LIQUID CRYSTAL" Function Material, December, 1988, in that it exhibits anisotropy of index of refraction or birefringence at relatively low temperature, i.e., lower than 30.degree. to 40.degree. C. and in that it exhibits isotropy thereof at a temperature higher than 40.degree. C. and thus causes the index of refraction not to be changed. Accordingly, in the case that the vari-focal liquid crystal lens is used as the first imaging optical system of the scanning optical apparatus, when the liquid crystal lens is exposed to a temperature higher than 40.degree. C., the liquid crystal lens can not vary the focal length thereof and thus makes it impossible to correct curvature of field in the direction of sub-scanning.
It is known in the art that an internal temperature of the scanning optical apparatus may be raised up to 40.degree. to a range of 50.degree. C. Thus, the above-mentioned problem may occur.